In general, a cast iron is an alloy of iron, carbon, and silicon, in which more carbon is present than can be retained in solid solution in austenite at the eutectic temperature. The amount of carbon in cast iron is usually more than 1.7 percent and less than 4.5 percent. There are many types of cast iron that are used in industry. Pig iron, which is the product of a blast furnace, can be considered cast iron since it is iron cast into pigs or ingots for later re-melting and casting into the final form. Another alloy iron is the austenitic cast iron, which is modified by additions of nickel and other elements to reduce the transformation temperature so that the structure is austenitic at room or normal temperatures. Austenitic cast irons are usually used for applications that require a high degree of corrosion resistance. White cast iron is a type of cast iron, in which almost all the carbon is combined with iron as cementite. White cast iron is typically used for applications that require a high abrasion resistance. Another class of cast iron is called malleable iron. Malleable iron is produced by annealing white cast iron to change the structure of the carbon in the iron. By annealing, the cementite within white cast iron is decomposed to small compact particles of graphite (instead of flake -like graphite seen in gray cast iron), increasing the ductility of the material. There are two other classes of cast iron, which are ductile and known as nodular iron and ductile cast iron. Nodular and ductile cast iron are made by the addition of magnesium or aluminum, which will either tie up the carbon in a combined state or will give the free carbon a spherical or nodular shape. This structure provides a greater degree of ductility or malleability to the casting. There are also alloy cast irons that contain small amounts of chromium, nickel, molybdenum, copper, or other elements added to provide specific properties. These alloys usually provide higher strength cast irons. One of the major uses for the higher strength irons is casting automotive crankshafts. These alloys are sometimes called semi-steel or proprietary names.
The most widely used type of cast iron is known as gray iron. Its tonnage production exceeds that of any other cast metal. Gray iron has a variety of compositions, but it is usually such that the matrix structure is primarily pearlite with many graphite flakes dispersed throughout. Gray cast iron has a very low ability to bend and low ductility. The low ductility is due to the presence of the graphite flakes which act as discontinuities. Gray cast iron has a number of material properties, such as low pouring temperature, high fluidity, low liquid to solid shrinkage, etc., that make it suitable for castings. Gray cast iron is also easily available, and is among the cheapest forms of ferrous material. One industrial application where gray cast iron has found widespread use is the automotive industry. The ability to tailor the properties of gray cast iron with alloying elements makes it suitable for different automotive parts. For instance, a composition of gray cast iron tailored for thermal fatigue resistance is used for engine blocks and cylinder heads, while another composition of gray cast iron tailored for high thermal conductivity and specific heat capacity is used for disk brake rotors.
To meet new performance requirements and more stringent automotive exhaust standards, engines are required to run hotter and at higher pressures than previously manufactured engines. These hotter temperatures and higher pressures require increased strength and enhanced thermal fatigue resistance of the gray iron castings. This is especially true of engine cylinder heads, which are most susceptible to thermal fatigue damage and creep due to their proximity to the combustion chamber. During combustion of fuel, gases in the combustion chamber may approach temperatures as high as 1300° F., and pressure as high as 160 MPa. This heat may be conducted to the cylinder head. To keep the heat from being transferred back to the suction air in the combustion chamber during the suction stroke (which reduces suction efficiency and, ultimately, engine efficiency), the cylinder head may be cooled by circulating a coolant through channels therein. This cyclic heating and cooling of the cylinder head during engine operation, combined with the high mechanical stresses due to pressure on the cylinder walls, makes the cylinder head highly susceptible to thermal fatigue and creep. Studies have shown that thermal fatigue and creep resistance of the gray cast iron cylinder head depends upon the composition of the alloying elements in the gray cast iron.
Traditionally, molybdenum (Mo) and vanadium (V) were known to be the most effective contributors for enhancing thermal fatigue resistance. These elements were considered to be unique among traditional alloying elements to produce refinement in eutectic cell size of gray cast iron that leads to enhanced thermal fatigue resistance. U.S. Pat. No. 5,242,510 (hereinafter the '510 patent) issued to Begin on Sep. 7, 1993, discloses a gray cast iron containing molybdenum to improve the high temperature thermal fatigue resistance of automotive components. The cast iron alloy disclosed in the '510 patent has a carbon content ranging from 3.4 percent to 3.6 percent by weight, a primary alloying addition of the combination of molybdenum in amounts varying from 0.25 percent to 0.4 percent, and copper in amounts varying from about 0.3 percent to 0.6 percent. The cast iron alloy of the '510 patent also contains silicon between about 1.8 percent to 2.1 percent and manganese between about 0.5 to 0.9 percent, with no more that 0.25 percent chromium and 0.15 percent sulphur. Samples cast from the iron alloy of the '510 patent exhibited a microstructure of a fully pearlite matrix having a refined eutectic cell size. The microstructure also exhibited a substantially uniform graphite distribution with random orientation. The flake size of graphite in the microstructure of the '510 patent was predominantly 5-7 ASTM. Samples cast from the alloy of the '510 patent also exhibited a tensile strength of at least 40,000 psi (≈276 MPa) and a hardness between about 179 to about 229 BHN.
Although the gray cast iron alloy of the '510 patent may have acceptable thermal fatigue resistance and strength, the cost of molybdenum containing gray cast iron alloy may be high. Increased cost of the gray cast iron material may, in turn, adversely impact the suitability of the material for automotive (and other commercial) applications. Thus, a lower cost gray cast iron alloy with good thermal fatigue resistance and strength is needed for commercial applications.
The present disclosure is directed at overcoming one or more of the shortcomings of the prior high strength gray cast irons.